PCI supports both 5V and 3.3V signalling. Most likely, the actual signalling rules are something like: For 5V signalling, 0 is less than .7 volts, 1 is more than 3 volts. For 3.3V signalling, 0 is less than .3 volts, 1 is more than 2 volts.
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David SchwartzDec 5 '11 at 19:43

wow, the ISA spec fits on a couple of pages. I wonder what the PCIe spec is in kilo grams.
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artistoexDec 5 '11 at 19:56

Regarding differential signaling: Is there a direct mapping of the levels of the regenerated difference signal onto 0 and 1 as with the unipolar scheme in PCI and ISA?
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artistoexDec 5 '11 at 20:18

The typical setup is that at least a .247 volt difference is required to indicate a legal value. The maximum a driver is permitted to output is a .454V difference. The average voltage of both differential outputs remains between 1.125V and 1.375V. Which line is at a higher voltage than the other determines if it's a 1 or a 0. (Of these buses, PCI-Express is the only one that is serial and the only one that uses differential signalling. You may need to double-check these values against different versions of PCI-Express, but it's about right.)
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David SchwartzDec 5 '11 at 20:36